1. Field of the Invention
The disclosed methods and systems relate generally to cement plant processes, and more particularly to cement plant finish mill control.
2. Background Information
Cement plant finish mills are an integral part of cement plants. As shown in FIG. 1, a typical arrangement has a finish mill 10 that is provided clinker 12 from a cement kiln and external sources, and may optionally be provided fringe (e.g., off specification clinker), limestone, water sprays, and other materials. Gypsum and/or natural anhydrite is added to regulate the setting time of the cement. The finish mill 10 grinds the feed mixture into very small particles 14 that are then fed to a separator 18 via a bucket elevator 16. The separator 18 uses cyclones with variable speed fans or adjustable vanes and/or a variable speed cage rotor to separate the fine product 20 from the larger particles 22 that need to be reground. The larger particles 22 that must be re-ground are fed back into the finish mill 10 as a “reject” or “re-circulating Load” stream, while the fine product 20 is pneumatically conveyed to a collecting cyclone and/or bag filter 24.
The continuous recycling of reject makes process variables in the mill 10 highly interactive and nonlinear, while the large capacity of the mill 10 makes their responsiveness to control corrections very sluggish. Long term variations in feed quality, mill charge, and equipment condition affect both the dynamic and steady-state behavior of these variables, and their stable operating limits. These combined characteristics make stable operation at or very close to the maximum production rate difficult to achieve through conventional control methods.
In a traditional method of cement finish mill (“mill”) control, an operator controls the fresh feed addition to the mill, where an underlying control strategy maintains the ratio of feeds (e.g., clinker, gypsum, limestone, fringe, etc.) to the mill in a proper proportion when the fresh feed changes. In such a method, the operator regularly monitors the finish mill (e.g., on a minute by minute basis) and makes the necessary adjustments. Operators thus must adjust the fresh feed input rate to mitigate problems with variations in feed hardness, mill choking, maximum bucket elevator load (e.g., power and/or motor current), cooling water limitations, changes in separator operation, mill loading constraints (e.g., sound and/or power), etc. The operators must also try to maximize the feed rate to improve the profitability of the cement plant. An operator rarely has time to continuously monitor a finish mill, and such a difficult control problem is generally not suited to manual operation/control.
Attempts at correcting the problems of manual operation/control led to the development of automated control systems for mills, where the total amount of feed, including both fresh feed and rejects, was determined by the automated control system 26 with some input from the operator. This method of course requires that a measurement of the reject flow is available. Using the automated system, an operator may establish a target total feed that is equal to the reject flow plus the fresh feed, and the control system adjusts the fresh feed up or down when the reject flow rate changes to maintain the total feed target. Such a system provides for automatic adjusting of mill operation when the feed hardness and/or the separator operation change. For example, if the feed gets harder, the particles exiting the mill are larger and the separator directs more material into the rejects stream. In response, the control system automatically reduces the fresh feed which allows the mill to grind the particles better. Unfortunately, changes in feed hardness, separator operation, and/or total feed target cause moderate to severe oscillations in the response of the finish mill, which is highly undesirable in processing plants and/or other control systems.
Fresh feed oscillation may be understood from an explanation of the process where, using feed hardness as an example, feed hardness changes at time zero minutes such that the mill gradually fills up with harder material and thus does not grind the particles as finely as needed. As a result, coarser material exits the mill to the bucket elevator and is fed to the separator which increases the rejects flow, which causes the control system to reduce the fresh feed. Reducing the fresh feed changes the grindability characteristics of the total feed, and the process repeats.
A variation of this control strategy includes controlling a “demand” that is expressed as a sum of (1) rejects and (2) x*(fresh feed), where x is greater than (1.0). This type of control dampens the oscillations, but provides sub-optimal control.